Apparatus for stereoscopic photography

ABSTRACT

An apparatus for providing left and right eye images along the axis of a single camera lens ( 7 ), the apparatus comprising optical means arranged to provide said left and right eye images ( 22, 20 ) as a pair of head-to-head or toe-to-toe images, i.e. with the two images ( 20, 22 ) separated by a center line and with either the tops of the two images ( 20, 22 ) or the bottoms of the two images ( 20, 22 ) adjacent the center line, so that the left and right eye images ( 22, 20 ) can be simultaneously recorded onto a single frame of a photographic film or other recording medium.

FIELD OF THE INVENTION

This invention relates to apparatus for providing and/or recording leftand right eye images for three-dimensional photography, cinematographyand/or videography and the means of viewing those images to produce afull stereoscopic effect.

BACKGROUND OF THE INVENTION

Three-dimensional photography is achieved by recording left and righteye views of a scene as two separate images. The left and right imagesare projected simultaneously onto a screen, typically one coated with ametallic surface. The separation between the two images is accomplishedby means of polarizing filters placed in the projection beams of theleft and right images with the polarizing axes set at 90  to each otherand therefore cancelling each other. To ensure that each eye sees onlythe corresponding image the stereoscopic scene is viewed through glasseswith polarizing filters whose polarizing axes are set at 90° to eachother. The resultant stereoscopic image can be viewed in full color.

The stereoscopic images can be recorded by various means, for example,using two cameras set side by side (as described in U.S. Pat. No.5,835,133) or a split lens system designed to record a stereoscopic pairof images on the same frame side by side or one image above the other.The latter scheme is known as an ‘over-under’ approach and an examplethereof is described in U.S. Pat. No. 4,436,369. The pair of images maybe arranged side by side, but with one image turned through 180°relative to the other image, as described in U.S. Pat. Nos. 5,357,369and 5,727,239.

An alternative technique has been developed for cinematography wherebythe discrete left and right eye images are recorded separately andsequentially on the image-recording medium, i.e. on alternate frames ofa photographic film at double the normal frame rate. Such an arrangementis shown in U.S. Pat. No. 4,676,298. The images can be viewed either byprojecting the images sequentially at double the normal frame rate or bysimultaneously projecting both the left and right eye images through anoptical arrangement that superimposes both images onto the screen.Crossed polarizing filters are placed in the projected beams of theappropriate images. The images are viewed through glasses withpolarizing filters whose polarization axes are at 90° to one anothercorresponding to the polarization axes of the projected images.

However, in the case of the “over-under” method, where a singleprojection lens and additional optical arrangements are used to projectboth images the projection arrangement has a detrimental effect that theoptical distortions introduced by the optical elements, mainly sphericalaberrations tend to be exaggerated when the two images are superimposedonto each other for viewing. A perfect match between the two images isnot possible on projection because each image is affected differently bythe spherical aberrations of the projection lens since both images arepart of the same image circle, i.e. the top corners of the upper imagereally match the bottom corners of the lower image instead of thecorresponding corners.

It would therefore be advantageous to provide apparatus for recordingand viewing three-dimensional images that has the advantages of thepresent known systems, but that mitigates the known disadvantages ofthese systems. For such an apparatus to be able to record and reproducea stereoscopic image with a realistic depth perception, it would need tohave all three of the following key elements:

a) a wide horizontal angle of view, of approximately 45° or 60°,

b) variable convergence, emulating the functions of human eyes, and

c) an inter-ocular distance corresponding to that of human eyes, theaverage distance in adults being 65 mm.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is providedan apparatus for providing left and right eye images with a horizontalangle of view of at least approximately 45° along the axis of a singlecamera lens, the apparatus comprising optical means arranged to providesaid left and right eye images as a pair of head-to-head or toe-to-toeimages, i.e. with the two images separated by a center line and witheither the tops of the two images or the bottoms of the two imagesadjacent the center line, so that the left and right eye images can besimultaneously recorded onto a single frame of a photographic film orother recording medium.

The optical means preferably comprises two reflecting elements for eachof the two images respectively, the reflecting elements being positionedin front of a camera lens. The reflecting elements conveniently comprisefirst and second reflecting elements arranged to receive the left andright eye images respectively and third and fourth reflecting elementsthat are arranged to receive said left and right eye images from saidfirst and second reflecting elements and to provide said left and righteye images along the axis of a camera lens, e. g. with a focal length of45 mm. on a 35 mm. stills format or a focal length of 28 mm on a 35 mm.cine format, resulting in a horizontal angle of view of approx 45°.

The first and second reflecting elements comprise plane mirrors and thethird and fourth reflecting elements preferably comprise two faces of atriangular prism, the plane mirrors each being arranged to reflect thelight rays of the left and right eye images respectively onto opposingfaces of the triangular prism, the triangular prism being arranged toredirect said incident light rays towards the camera lens whose axis isat 90° to the direction of view of the first and second reflectingelements.

The apparatus preferably includes variable convergence adjustment means,so that the convergence of the optical axes of the left and right eyeimages can be varied in a manner substantially consistent with theconvergence function of natural eyes. Preferably, the convergenceadjustment means comprises a mechanism for varying the angle at whichthe first and second reflecting elements are set in relation to thedirection of view and consequently to the axes of the third and fourthreflecting elements. The convergence adjustment means may comprise arotary adjustment mechanism. The first and second reflecting elementsare preferably interconnected by a mechanical linkage, such that thefirst and second reflecting elements are constrained to being arrangedat the same angle of convergence relative to the direction of view.

The apparatus preferably includes variable inter-ocular adjustment meansarranged to adjust the distance between the image axes of the said firstand second reflecting elements such that they are separated by adistance substantially consistent with the average inter-ocular distanceof human eyes, i.e. 65 mm. being the average in adults.

The apparatus preferably also includes an optical element arranged toextend the horizontal angle of view of the apparatus to approximately60°. Preferably, the optical element comprises a pair of opticallyidentical first lenses or lens groups of negative optical power, eachfirst lens or lens group being located along the respective axes of theleft and right eye images and in front of said first and secondreflecting elements. The optical element preferably also comprises asecond lens or lens group of positive power, located along the axisbetween the camera lens and third and fourth reflecting elements suchthat both left and right eye images are incident on the second lens orlens group. A suitable spacer, such as an extension tube placed betweenthe camera body and the camera lens, may be used in place of the secondlens or lens group.

The first lens group preferably comprises a pair of individual halves ofan optical element, each half optical element being placed in front ofsaid first and second reflecting elements in correct alignment with theoptical axis of the camera lens.

Both halves of the first optical element are preferably coupled to theconvergence adjustment means, so that operation of the convergenceadjustment means causes adjustment of the two halves of the firstoptical element.

As mentioned above, the first and second reflecting elements maycomprise plane mirrors and the third and fourth reflecting elements maycomprise a triangular prism with reflecting sides, the plane mirrorseach being arranged to reflect the light rays of the left and right eyeimages respectively onto opposing faces of the triangular prism, and thetriangular prism being arranged to redirect said incident light raystowards the camera lens.

The second lens or lens group of positive power may be replaced oraugmented by parabolic mirrors placed in the position (and in place) ofthe reflecting prism surfaces.

The first and second lenses or lens groups may be of sphericalconstruction. Additionally or alternatively, the first and second lensesor lens groups may be replaced or augmented by anamorphic lenses.

The optical apparatus is preferably arranged so that it can be coupledto the front of a standard lens of a camera or other image-recordingdevice.

According to a second aspect of the present invention there is providedapparatus for use in viewing a three-dimensional image, the apparatuscomprising optical means arranged to simultaneously receive left andright eye images in the format of a pair of head-to-head or toe-to-toeimages and to provide a superimposed projected image comprising the leftand right eye images to a viewing screen.

The optical means preferably comprises first and second reflectingelements arranged to respectively project the left and right eye images,the first and second reflecting elements being arranged along an axisextending substantially at right angles to the axis of the projectionlens.

An inter-ocular adjustment means is preferably provided to adjust theseparation of the image axis of the first and second optical elements.For standard projection purposes, the inter-ocular distance between theaxes of the left and right images does not have to be the inter-oculardistance of human eyes.

The apparatus may also include means for adjusting the angle ofincidence of said first and second reflecting elements to the centralaxes of the respective left and right eye images, so that theconvergence of said left and right eye images is adjustable.

The optical apparatus is preferably arranged so that it can be coupledto the front of a projection lens.

The apparatus may further include polarizing means for polarizing theleft and right eye images, whereby the superimposed composite image maybe viewed using corresponding polarizing filters to perceive athree-dimensional image when projected onto a screen with a suitablemetallic surface.

The apparatus may further include a viewing device comprising a viewingbox into which said composite image is projected, the viewing box havingone or more reflective surfaces that are arranged to project the leftand right images onto a side wall of said viewing box, the viewing boxfurther comprising a viewing window through which said projected imagemay be viewed.

An optical element may be provided on the side wall of the viewing boxon which the left and right images are projected, the optical elementbeing arranged to reflect incident light rays back along their axes sothat a three-dimensional image may be viewed with the naked eyes by asingle viewer without the use of polarizing elements. Preferably, theoptical element may comprise front-projection material. The opticalelement or front-projection screen may be placed at a distance, i.e.remote from the viewing box.

An identical apparatus attached to the front of a camera lens and placedin the position of the viewer in correct alignment along thecorresponding axes of the projection apparatus will be able to recordthe left and the right eye images reflected from the auto-collimatingfront-projection screen simultaneously. Moreover, a subject placed infront of the front projection screen within the field of view of theapparatus and lit appropriately will be recorded as a three-dimensionalcomposite image, i.e. it will be seen by the camera as being inside theprojected image. This can be particularly effective when thestereoscopic projected image has some foreground elements and the“subject” is placed at middle distance. Front-projection is anestablished technique for creating visual effects composites inconventional monocular image recording.

The same viewing box can be used to view three-dimensional imagesrecorded by means of the first aspect of the present invention andreproduced in the from of photographs, television or computer images.The recorded image in the head-to-head or toe-to-toe configuration maybe transferred by means of an arrangement of lenses acting as a relay toan apparatus as described above and projected as a composite image ontothe screen. A three-dimensional video image can also be viewed in realtime when a device as described above is attached to the video cameralens.

The viewing box can also be used for direct three-dimensional viewing (athree-dimensional periscope) utilizing the relays system with anapparatus attached at each end of the viewing box, the first apparatusacting as a taking system that creates the left and right images and thesecond apparatus acting as a projection system that superimposes the twoimages onto the viewing screen inside the box. An image intensifier orsimilar device may also be placed in the image path.

The apparatus may further include a second viewing device that isattached to the flip-out screen of a video camera enabling the viewer tosee a three-dimensional image during the recording or playback of a pairof stereoscopic images when the apparatus described above is attached tothe front of the video camera. The same viewing device may also be usedindependently of the video camera and in conjunction with a flat screenTV or computer monitor. When the apparatus described above is used inconjunction with a cine camera, an image from a video assist camera,which is typically attached to the viewfinder, can be viewed by means ofthe second viewing device at the time of the recording and also forplayback.

Advantages of the apparatus of the present invention include the singlecamera system, variable convergence control mimicking the movements ofthe human eyes in the horizontal plane, wide horizontal angle of view(either 45° or 60°) at the correct inter-ocular distance correspondingto that of the average human eyes.

Another advantage of the apparatus of the present invention is that thecamera lens does not compound the spherical aberrations of the recordedimages, because the left and right eye images are recorded ashead-to-head or toe-to-toe images and, as such, are affected equally bythe spherical aberrations. Thus, when the two images are superimposedfor viewing they effectively cancel one another. The quality of theperceived three-dimensional image is therefore improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates apparatus according to an embodiment ofthe present invention;

FIG. 2A schematically illustrates a single photographic frame on whichleft and right eye images have been recorded in a horizontal format anda head-to-head configuration by apparatus as shown in FIG. 1;

FIG. 2B schematically illustrates a single photographic frame on whichleft and right eye images have been recorded in a vertical format and ahead-to-head configuration by apparatus as shown in FIG. 1;

FIG. 2C schematically illustrates a single photographic frame on whichleft and right eye images have been recorded in a horizontal format anda toe-to-toe configuration by apparatus as shown in FIG. 1;

FIG. 2D schematically illustrates a single photographic frame on whichleft and right eye images have been recorded in a vertical format and atoe-to-toe configuration by apparatus as shown in FIG. 1;

FIG. 3 schematically illustrates the optical paths of light rays throughthe apparatus shown in FIG. 1;

FIG. 4A illustrates the method of adjustment of the inter-oculardistance for the apparatus shown in FIG. 1;

FIG. 4B illustrates the coincidence of a 60° horizontal angle of viewwith the binocular field;

FIG. 5A schematically illustrates the location of first and second afocal adjustment lenses according to an embodiment of the presentinvention;

FIG. 5B schematically illustrates the optical configuration of the firsta focal lenses of the arrangement shown in FIG. 5A;

FIG. 6 illustrates a viewing device in accordance with the presentinvention;

FIG. 7 schematically illustrates the light paths within the viewingdevice shown in FIG. 6;

FIG. 8 is a further view of the viewing device shown in FIG. 6 togetherwith the apparatus shown in FIG. 1 with a video camera attached;

FIG. 9A schematically illustrates the apparatus shown in FIG. 1 with anattached video camera and the location of a viewing device over theflip-out screen;

FIG. 9B schematically illustrates the optical layout and location of aviewing device relative to the flip-out screen of the video camera andthe optical layout of the apparatus shown in FIG. 1; and

FIG. 9C schematically illustrates the location and the optical path ofthe viewing device relative to a flat screen monitor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The apparatus 1 shown in FIG. 1 is arranged to be coupled to aconventional camera 3 by means of a coupling ring 5. The camera 3 may bea conventional 35 mm. camera or the like, or may equally be a video orcinematographic camera. The camera 3 has a lens 7 to which the couplingring 5 of the apparatus 1 is attached, for example, utilising the screwthread provided on conventional camera lenses. Located along the axis ofthe camera lens 7 and coupling ring 5 is a triangular prism 9 arrangedso that two of the prism faces are directed towards the camera lens 7.The prism faces are coated and act as front-surface plane mirrors.Alternatively small front-surface mirrors can be attached to the facesof the prism.

Located on the same plane as the triangular prism 9, and therefore atsubstantially 90° to the camera axis, are two plane mirrors 11 and 12.An aperture 14 is provided in the front face of the apparatus 1 throughwhich light rays are received. The plane mirrors 11 and 12 are arrangedat an angle to the triangular prism 9 such that light rays receivedthrough the aperture 14 are reflected onto respective faces of thetriangular prism 9. The light rays received from a scene by the planemirror 12 constitute a left eye image, whereas the light rays receivedby the plane mirror 11 constitute a right eye image.

The prism 9 redirects the left and right eye images to the camera lens 7such that they are recorded on the focal plane 16 of the lens 7 (seeFIG. 3) at which an image recording medium is located. Both left andright eye views are recorded on a standard frame divided into two alongthe horizontal line. The camera 3 is positioned so that its lens axis isat 90° to the direction of view of the apparatus. Thus, when the cameralens 7 is pointing down, the left and right eye images are recorded in ahead-to-head configuration and in toe-to-toe configuration when thecamera lens 7 is pointing up.

FIGS. 2A, 2B, 2C and 2D schematically illustrates the format in whichthe left and right eye images are recorded. FIG. 2A illustrates acomplete photographic frame 18. On the lower half of the frame 18 isrecorded the right eye image 20 of a scene and on the upper half of theframe 18 is recorded the left eye image 22 of the same scene. It can beseen that the left and right eye images are recorded as head-to-headimages, by which is meant that the elements of one image appear to beupside down with respect to corresponding elements of the other image.However, it will be noted that the left and right images are not simplyrecorded as mirror images along a line of symmetry intersecting theframe 18, but are laterally reversed with respect to one another. Thestandard frame can be split either along the horizontal center line asillustrated in FIG. 2A or with the camera 3 rotated 90°, the frame canbe split along the vertical center line as in FIG. 2B. The horizontalsplitting arrangement has the advantage of a wider horizontal angle ofview. In both cases, the images are rectangular in shape with thehorizontal axis longer than the vertical one. FIGS. 2C and 2D illustratethe toe-to-toe configuration.

FIG. 3 schematically illustrates the optical path of light rays throughthe apparatus shown in FIG. 1. The scene 24 includes a subject S onwhich the apparatus is converged. To aid clarity, only the center lightrays of the left and right eye views 27L and 27R are illustrated. Byfollowing the illustrated light rays 27L and 27R, it will be seen thatthe reflecting surfaces of the prism 9 cause the separate left and rightviews to be re-directed to the camera lens located at X. Images of theleft and right eye view 24′ and 24″ are formed on the focal plane 16.

Each plane mirror 11, 12 is mounted on an adjustment mechanism (notshown) such that some degree of rotational adjustment about the verticalaxes of the mirrors 11, 12 is possible. This allows the angle that themirrors 11, 12 subtend to the faces of the prism 9 to be adjusted. Thisalso adjusts the angle of convergence of the optical axes 27L and 27R.The adjustment mechanisms of the two mirrors 11 and 12 are preferablycoupled to one another such that a single adjustment control can beutilised to adjust the positions of both mirrors 11 and 12, and bothmirrors 11, 12 are adjusted in synchronism.

One method of providing the adjustment facility is to mount the mirrors11 and 12 onto two meshing gears (not shown) so that the reflectingsurface of each mirror 11, 12 runs through the pivot point of the gear.As one gear is rotated in one direction it will automatically rotate theother gear in the opposite direction. However, it will be appreciatedthat other adjustment mechanisms may also be used.

It has been found that the angle of convergence is particularlyimportant in order to emulate the eyes of an observer standing at thesame position as the camera 3 at the time of image capture.

The distance between the image axes of the plane mirrors 11 and 12 isalso preferably adjustable. This distance is known as the inter-oculardistance and it has been found that, for a realistic three-dimensionalimpression to be gained, the inter-ocular distance must be substantiallythe same as that of the average distance between human eyes. Thisdistance is generally between 60 and 75 mm. and it is therefore thisdistance and range of adjustment that is preferably provided in theapparatus 1.

Referring next to FIG. 4A, when the plane mirrors 11 and 12 are movedtogether along their respective axis from position A to position B,preferably in alignment with the longitudinal axis of the prism 9, theimage center axes intercepted by the surfaces of the mirrors 11 and 12are at a different point, resulting in the intra-ocular distance,represented by arrow b, at position B being greater than that atposition A, represented by arrow a.

A horizontal angle of view of approx 45° can be covered by a camera lensof 45 mm. focal length on full 35 mm. stills format and by a camera lensof 28 mm. focal length on a 35 mm. cine format.

FIG. 4B shows the binocular field—the overlap area of the left and righteye monocular fields. Concentric circles represent the angles of view.It can be seen that the 60° circle corresponds best to the horizontalcoverage of a rectangular image frame with aspect ratios of 2.5:1 (A)and 3:1 (B). Therefore, an ideal stereoscopic lens imaging apparatusshould have a horizontal angle of view of approximately 60° in order tosubstantially match this natural horizontal field of vision.

In the arrangement shown in FIG. 5A, a horizontal angle of view ofapproximately 60° is achieved by placing a lens or lens group 32 ofpositive power in the path of the light rays between the prism 9 and thelens 7 of the camera 3 and placing a further lens or lens group 34 and34′ of negative power in the path of the light rays forming the left andright eye images prior to the light rays being incident on the planemirrors 11 and 12. Because the light rays from the left and right eyeimages respectively only form one half of the total image on the focalplane 16, the lens groups placed before each of the plane mirrors 11 and12 may be of the split-lens type to best match the opticalcharacteristics of the primary imaging lens 7 on the camera 3. This isillustrated schematically in FIG. 5B where the lens L is cut in halfalong the center line C, and separate half moon lenses 34 and 34′ areground to the required rectangular shape and located in front of theleft and right plane mirrors 11, 12 respectively. The sphericalcharacteristics of the lens L are indicated by concentric circles inFIG. 5B.

The apparatus required to view the recorded images so as to experiencethe three-dimensional impression is dependent upon the recording mediumused to record the initial left and right eye images. If the left andright eye images are recorded on conventional photographic transparencyfilm or as a moving image by a cine camera, then the three-dimensionalimage may be viewed by attaching the apparatus of FIG. 1 to the lens ofa conventional transparency projector, placing polarizing filters overthe left and right halves of the aperture 14 of the apparatus 1, so asto cross-polarize the left and right eye images, projecting thepolarized images onto a metallic screen and viewing the projected imageusing correspondingly polarized filters or glasses. The use of thepolarizing filters and polarizing glasses ensures that the correspondingeye of an observer only receives the corresponding projected left orright eye image. Ideally, the projection version of the apparatus 1would be modified to suit the particular projection lens.

Viewing of the recorded left and right images when they have beenrecorded on print film or video tape or a live feed from a video cameranecessitates the use of a further viewing device. A part of thenecessary further viewing device is shown in FIG. 6. The viewing deviceessentially comprises a box 36 that has two apertures formed in adjacenttwo sides of the box 36. A first aperture 38 is used to introduce lightrays from the recorded left and right eye images as will be described inmore detail below. Located within the box at an angle to each of thesides is a two-way mirror 40. The two-way mirror is located so as todivide the box into two halves, with the first aperture 38 being locatedin one half and a second aperture 42 in the other half. In use, thelight rays from the left and right eye images are directed through thefirst aperture 38 so that they are incident upon the reflective side ofthe two-way mirror 40. The mirror 40 is angled such that the left andright images are redirected onto a front-projection screen attached tothe inside surface of the rear wall 44 of the box 36.

As shown in FIG. 7, the projected image is viewed by an observer throughthe second aperture 42, looking through the two-way mirror 40. With thebox wall 44 removed, a front-projection screen 44A can be placed at somedistance away from the two-way mirror 40.

The front projection screen 44A is covered by a material that isauto-collimating. It comprises a plurality of tiny spheres that have theoptical property that they redirect light that is incident on them backalong the axis of the light ray. The effect of this property is that theleft and right eye images projected onto the rear wall 44 or on thescreen 44A by the two-way mirror 40 are reflected in a highlydirectional manner to an observer looking through aperture 42. Thismeans that, if the observer is positioned correctly in front of theaperture 42, the left eye image will only be seen by the left eye of theobserver and, equally, the right eye image will only be seen by theright eye of the observer. This therefore allows the observer to see thethree-dimensional image without the use of polarizing filters, i.e. withthe naked eyes.

FIG. 8 shows a camera with a stereoscopic apparatus 1C attached to thecamera lens placed in the position of the viewer and aligned correctlyalong the corresponding axis of the stereoscopic projection apparatus 1Pso that it can record both left and right stereoscopic imagessimultaneously. Moreover, a subject S placed in front of thefront-projection screen within the field of view of the apparatus 1C andlit appropriately by light source LS will be recorded as athree-dimensional composite image with the background.

FIG. 9A illustrates a second viewing device VD attached to the flip-outscreen 45 of a video camera 3 displaying a stereoscopic pair of images20 and 22. The video camera lens 7 is attached to the stereoscopicapparatus 1 by means of a ring 5 resulting in a pair of stereoscopicimages at focal plane 16. The viewer can see a three-dimensional imagethrough eyepieces 46′ and 46″. The direction of view is indicated by anarrow pointing towards the subject S. The stereoscopic scene can beviewed during recording and playback.

The viewing device may also be used remotely or independently of thevideo camera in conjunction with a flat screen TV or computer monitor.

FIG. 9B is a further illustration of the video viewer shown in FIG. 9A.

FIG. 9C illustrates the light paths 47′ and 47″ from the center of theleft and right stereoscopic images 20 and 22 displayed on a flat screenmonitor 45 via reflecting surfaces M1, M2, M3 and M4 to the eyepieces46′ and 46″.

1. A stereoscopic attachment for a camera or projector for providingleft and right eye images with a horizontal axis of view ofapproximately 45° along the axis of a single camera lens, the apparatuscomprising: optical means arranged to provide said left and right imagesas a pair of head-to-head or toe-to-toe images such that the left andright eye images are simultaneously recorded as a composite image onto asingle frame of a recording medium; said optical means including tworeflecting elements for each of the two images respectively with thereflecting elements being positioned in front of a camera lens; saidreflecting elements including first and second reflecting elementsarranged to receive the left and right eye images respectively andfurther including third and fourth reflecting elements arranged toreceive said left and right eye images from said first and secondreflecting elements to provide said left and right eye images along theaxis of a camera lens whose axis is set at 90° to the direction of viewof said first and second reflecting elements; said first and secondreflecting elements comprising first and second plane mirrors and saidthird and fourth reflecting elements comprising two smaller planemirrors positioned adjacent to each other; said first and second planemirrors each being arranged to reflect the light rays of the left andright eye images respectively onto the smaller plane mirrors which arearranged to reflect the incident light rays towards said camera lens; anoptical element arranged to extend the horizontal angle of view of theapparatus, the optical element comprising a pair of optically identicalfirst lenses or lens groups of negative optical power, each first lensor lens group being located along the respective axes of the left andright eye images and in front of the said first and second reflectingelements; the optical element also comprising a second lens or lensgroup of positive power located along the axis between the camera lensand the third and fourth reflecting elements such that both the left andright eye images are incident on the second lens or lens group; bothparts of the optical element being coupled to a convergence adjustmentmeans so that operation of said convergence adjustment means causesadjustment of the two parts of said optical element; and a viewingdevice comprising a viewing box into which said composite image isprojected, the viewing box having a wall and one or more reflectivesurfaces that are arranged to project the left and right eye images ontosaid wall, the viewing box further comprising a viewing window throughwhich said projected composite image may be viewed.